1. Field of the Invention
This invention relates to fluid pressure servomotors of the type used to operate power brakes and the like, and particularly to a sealing mechanism for maintaining an airtight seal for a sliding portion of a hub member or the like.
2. Description of the Prior Art
As shown in FIG. 6, a fluid pressure servomotor of the type disclosed in U.S. Pat. No. 4,345,506 includes a partition wall 123 has a centrally located opening 123a, a sealing member 128 made of rubber and arranged in the opening 123a, a ring bearing 129 made of plastic material for axially slidably guiding a cylindrical portion 141A of hub member 141. The ring bearing member is arranged in the opening 123a. A retainer 191 is partially inserted in the sealing member 128 and pressed in the opening 123a together with the sealing member 128 and presses the ring bearing 129 to a flange portion 123b of partition wall 123 so as to retain the sealing member 128 and the ring bearing 129 in the opening 123a. A lip portion 128a of sealing member 128 is pressed into tight engagement with the cylindrical portion 141A of hub member so as to maintain an airtight sealing engagement between the lip portion 128a and the cylindrical portion 141A. However, in the foregoing sealing mechanism, the ring bearing 129 of plastic material shrinks in both radial and axial directions due to thermal hysteresis. As a result, the ring bearing 129 moves in relation to the sealing member 128. Also, the pressing force of the sealing member 128 of rubber to the cylindrical portion 141A of hub member decreases due to thermal deterioration of the sealing member 128. In this condition, the ring bearing and the sealing member 128 cannot be maintained in a co-axial position and the sealing member 128 and the cylindrical portion 141A guided by ring bearing 129 are likewise not maintained in a co-axial position. As a result, one portion of the sealing member 128 is pressed against the portion 141A with increased force and an opposite portion of the sealing member 128 will be loosely engaged or completely disengaged from the portion 141A. Consequently, the airtight engagement of the lip portion 128a of sealing member 128 with the portion 141A is interrupted and the performance of the servomotor decreases.